A typical example of the heat accumulator of the type concerned is a hot water heat accumulator disclosed in, for example, Japanese Utility Model Laid-open Publication (JP-UM-A) No. 5-25254.
As shown in FIG. 8 hereof, the disclosed hot water heat accumulator 100 includes an outer vessel 101 and an inner vessel 103 disposed in the outer vessel 101 with a vacuum part 102 interposed between the outer and inner vessels 101 and 103. The heat accumulator 100 further includes an inlet pipe 104 for introducing water into the inner vessel 103, a plurality of heat transfer tubes 106 embedded in a heat storage material 105 for the passage therethrough of the water which is introduced from the inlet pipe 104, and an outlet pipe 107 for discharging water out of the hot water heat accumulator 100 after the water has passed through the heat transfer tubes 106.
With the hot water heat accumulator 100 thus arranged, while the water held within the inner vessel 103 is at a high or elevated temperature, the heat storage material 105 absorbs and stores thermal energy from the water.
Alternatively, while the water held within the inner vessel 103 is cold, the heat storage material 105 releases heat to the water to thereby warm the water inside the inner vessel 103.
The most part of the interior of the hot water heat accumulator 100 is surrounded by the vacuum part 102 so that heat dissipation from the inside to the outside of the hot water heat accumulator 100 can be avoided. However, since a portion of the inlet pipe 104 projecting outwardly from the outer vessel 101 is free from thermal insulation, the inlet pipe 104 can form a thermal passage, allowing heat to dissipate or leak from the inside to the outside of the hot water heat accumulator 100. With this heat dissipation, a temperature drop will occur at an area indicated by phantom lines 108 shown in FIG. 8, leading to reduction of heat retention property of the hot water heat accumulator 100 caused due to an undesired internal temperature drop. Similarly, the outlet pipe 107 can also form a thermal passage and allows heat to dissipate or leak from the inside to the outside of the hot water heat accumulator 100. This may cause a temperature drop to occur at an area indicated by phantom lines 108 shown in FIG. 8, which will lower the heat retention property of the hot water heat accumulator 100 due to an undesired internal temperature drop.
Thus, the conventional hot water heat accumulator has a room to improve the heat-retention property.